The present invention relates to a super-resolution optical head apparatus for optically reading information from an information storage surface with a light beam focused thereon. More particularly, it relates to a super-resolution optical head apparatus applicable to an optical head apparatus for optical disks, a bar-code scanner, and an image scanner.
The super-resolution optical head apparatus mentioned above comprises: a focusing means for focusing a coherent beam onto a fine spot on the information storage surface; and a photoelectric converting means for detecting the intensity of the beam reflected from the information storage surface. The above super-resolution optical head apparatus is equipped with various arrangements that have been devised to obtain a fine focal spot equivalent to or smaller than the diffraction limited.
FIG. 20(a) is a schematic view showing the structure of a conventional image-forming optical system using an annular diaphragm, which is well-known as a super-resolution optical system, as a double diffraction optical system. Such a super-resolution optical system using the annular diaphragm or a phase plate has found applications in optical head apparatus, which are reported in the following documents:
(1) "High Density Optical Recording by Super Resolution," Y. Yamada, Y. Hirose and K. Kubota, Proc. Int. Symp. on Optical Memory. 1989, Jap. J. of Appl. Phys., Vol. 28 (1989) supplement 28-3, pp. 197-200.
(2) "Optical Head with Annular Phase-Shifting Apodizer," Hideo Ando, Tsuneshi Yokota and Koki Tanoue, Jap. J. Appl. Phys., Vol. 32 (1993) pp.5269-5276, pt. 1, NO. 11B.
As shown in FIG. 20, a coherent beam emitted from a coherent light source 50 is turned into parallel-beams upon passing through a collimator lens (a first Fourier transform lens) 51. The resulting parallel beams are then allowed to pass through apertures 52a and 52b (slits in one dimension) of an annular diaphragm 52 and converged by an objective lens (a second Fourier transform lens) 53 so as to form an image, thereby providing a super-resolution spot having the intensity distribution I(X), which is shown in FIG. 20 as the power spectrum of the transmittance of the foregoing annular diaphragm 52.
The above document (1) discloses an optical head which forms such a super-resolution spot in one dimension and uses only the main lobe thereof obtained by means of knife-edges constituting a slit. The above document (2) discloses a system which uses a plurality of phase distributions and a specified amplitude distribution as the annular diaphragm in order to form a super-resolution spot in two dimensions, thereby suppressing the side lobes on both sides of the main lobe shown in FIG. 20(b). In the system, the conditions for designing the annular diaphragm are optimized to suppress the side lobes.
However, the system for suppressing the side lobes by means of the annular diaphragm is not free from a reduction in intensity of the focused beam. In the case where the peak intensity of the focused beam is reduced to about 50% to 15%, e.g., if the half bandwidth of the main lobe is reduced to 85% of the diffraction limited, the intensity of the side lobe becomes about 7% of the peak intensity of the main lobe.
As described above, if the aperture through which light is incident upon the objective lens is formed into a slit or an annularity, there can be achieved super resolution smaller than the diffraction limited with the side lobes suppressed to a certain extent. On the other hand, there arise the following problems: (1) since the quantity of light reaching an image forming plane is reduced significantly, the quantity of light in the main lobe is also reduced disadvantageously; (2) moreover, since the aperture for shielding the side lobes is provided a higher accuracy is required in adjusting the optical path, while the reliability of the apparatus is lowered because the components of the optical system deteriorate with the passage of time or for other reasons; and (3) furthermore, the half bandwidth of the beam is reduced to about 90% to 80% at most.